1. Field of the Disclosure
The present disclosure relates to a method of forming an inflated aerofoil, and in preferred implementations is particularly suitable for forming an aerofoil for use as a blade or a vane in a gas turbine engine.
2. Description of the Related Art
Gas turbine engines comprise compressor and turbine arrangements having alternating stages of rotating aerofoil blades and stationary aerofoil vanes. A number of axial flow compressors are often provided which each supply high pressure air either to another downstream compressor or to a combustor. It is usual for outlet guide vanes (OGVs) to be provided aft of each compressor in order to straighten the flow from the compressor and direct it appropriately to another downstream compressor or to the combustor. The outlet guide vanes are also provided in the form of stationary aerofoils.
In order to reduce engine weight, particularly for gas turbine engines used to power aircraft, it is conventional to form these aerofoil blades and vanes so that they have a hollow configuration, and this is achieved by inflating them from planar pre-forms using a super plastic forming process which can be performed subsequent to, or simultaneously with, a hot creep forming technique to achieve the overall aerofoil shape from the planar pre-form. It is usual to cut the pre-form to define the leading and trailing edges of the aerofoil to be formed, and to create a service lug at each end of the pre-form to mount and locate the pre-form during the hot creep forming and inflation steps.
As propulsive gas turbine engines for aircraft have become larger over recent years, so have a number of the aerofoils used inside them; particularly the outlet guide vanes, and problems have been experienced in reliably and efficiently manufacturing the larger aerofoil shapes. One such problem is that of so-called “spring-back”, whereby an intermediate and unfinished aerofoil created via the hot creep forming and inflation process subsequently loses its intended shape when it is cut and removed from the service lugs during finishing to prepare the final aerofoil.
FIG. 1 shows a chordal cross-section through an intermediate and unfinished aerofoil 1 following the hot creep forming and inflation process, but before the service lugs at each end of the aerofoil have been removed during a subsequent finishing process. The intermediate aerofoil is created to have a desired degree of curvature to both its concave pressure side 2 and its convex suction side 3. However, FIG. 2 shows the chordal cross-section which can result following removal of the service lugs. As will be noted, the degree of curvature present in the intermediate aerofoil has relaxed or “sprung-back” following removal of the service lugs, thereby changing the shape of the aerofoil such that it no longer conforms to its design shape.
In order to address this issue, it has previously been proposed to perform the hot creep forming process in two discrete stages. Whilst this has been effective, to a certain degree, in eliminating or reducing the likelihood of spring-back occurring, it is a very inefficient manufacturing technique which requires additional time and power when compared to a single stage process.